Normal filming, usually at a rate of 18 frames per second, can be carried out only as long as there is sufficient illumination for proper exposure within the available range of diaphragm apertures. In order to permit the use of the camera when there is insufficient illumination even at maximum aperture, or at a more restricted diaphragm setting required for a desired depth of field, it has already been proposed to extend the exposure time of a frame by temporarily deactivating the shutter; see commonly owned U.S. Pat. No. 4,003,646. According to that prior patent, the intensity of the incident light is measured by a photosensor disposed in a separate light path.
Cinematographic cameras are known in which a photosensor, designed to control the diaphragm aperture, responds to incident light from the picture-taking objective which is reflected from the objective axis by a surface of the shutter lying in line with the image gate of the camera in the exposure-blocking position thereof. Such an arrangement has the advantage of making the amount of light reaching the photosensor exactly proportional to the intensity of the beam passing the image gate during exposure, even in the case of a varifocal objective, inasmuch as the field of view is the same in both instances. The light pulses falling upon the photosensor during successive cycles give rise to voltage pulses which must be integrated to provide a suitable control voltage; proper adjustment, therefore, necessitates a constant shutter speed. Such a system, accordingly, would be ineffectual in the case of long-term exposures in which the shutter is arrested for longer or shorter periods.